786 The Bight Eon, Lord Bayleigh [June 10, 



WEEKLY EVENING MEETING, 



Friday, June 10, 1898. 



SiE William Huggins, K.C.B. D.C.L. LL.D. F.R.S. Vice-President, 



in the Chair. 



The Eight Hon. Lord Eayleigh, M.A. D.C.L. LL.D. F.R.S. M.B.I. 



Professor of Natural Philosophy, B.I. 



Some Experiments with the Telephone. 



Early estimates of the minimum current of suitable frequency 

 audible in the telephone having led to results difficult of reconcilia- 

 tion with the theory of the instrument, experiments were undertaken 

 to clear up the question. The currents were induced in a coil of 

 known construction, either by a revolving magnet of known mag- 

 netic moment, or by a magnetised tuning-fork vibrating through 

 a measured arc. The connection with the telephone was completed 

 through a resistance which was gradually increased until the residual 

 current was but just easily audible. For a frequency of 512 the 

 current was found to be 7 X 10~^ amperes.* This is a much less 

 degree of sensitiveness than was claimed by the earlier observers, 

 but it is more in harmony with what might be expected upon 

 theoretical grounds. 



In order to illustrate before an audience these and other experi- 

 ments requiring the use of a telephone, a combination of that 

 instrument with a sensitive flame was introduced. The gas, at a 

 pressure less than that of the ordinary supply, issues from a pin- 

 hole burner | into a cavity from which air is excluded (see figure). 

 Above the cavity, and immediately over the burner, is mounted a 

 brass tube, somewhat contracted at the top where ignition first 

 occurs. J In this arrangement the flame is in strictness only an 

 indicator, the really sensitive organ being the jet of gas moving 

 within the cavity and surrounded by a similar atmosphere. When 

 the pressure is not too high, and the jet is protected from sound, the 

 flame is rather tall and burns bluish. Under the influence of sound 

 of suitable pitch the jet is dispersed. At first the flame falls, 



* The details are given in ' Phil. Mag.' vol. xxxviii. p. 285 (1894). 

 t The diameter of the pin-hole may be 0* 03". 

 X ' Camb. Proc.' vol. iv. p. 17, 1880. 



